Personally, I don't think it was a "just in case" situation. Sera has apparently always seen Dean as a metaphor for Michael, whether angels were ever going to be in the mytharc or not. I think it was a fortuitous little bit of story that ended up meaning more in the overall series than originally intended.
That happens with good writing--when things just fall together like that. (I'm not saying the final execution of the angel stuff was good. I think they forced a lot of it, which isn't good. But I hope you understand what I'm saying.)
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Personally, I don't think it was a "just in case" situation. Sera has apparently always seen Dean as a metaphor for Michael, whether angels were ever going to be in the mytharc or not. I think it was a fortuitous little bit of story that ended up meaning more in the overall series than originally intended.
That happens with good writing--when things just fall together like that. (I'm not saying the final execution of the angel stuff was good. I think they forced a lot of it, which isn't good. But I hope you understand what I'm saying.)